In known drilling operations of the type described generally above, a pilot hole is drilled along an inverse arcuate path by means of a rotary drill bit powered by a drill string having a bent sub connected therein. The drill can be "walked" right or left, or up or down by controlling the orientation of the bent sub, and varying the thrust on, and the rpm of, the bit. Once a pilot hole has been established between locations on opposite sides of the obstruction, a reamer that advances concentrically along the drill pipe is used to enlarge the hole, with the reamer being followed by a liner string of casing that acts to prevent collapse or cave-in of the borehole walls. When the following liner is completely in place, the drill pipe is withdrawn from the borehole to provide an unobstructed passage.
Although the foregoing procedure has been widely used, it has the principal disadvantage that a plurality of passes is required to complete the borehole, with attendant difficulties and expense. A long, open excavation is required at the entrance end to enable welding the entire liner string together, since it must be pulled through the hole behind the reamer. Moreover, the manner that has been employed to control the direction of the hole involves a degree of guesswork coupled with repeated "single-shot" directional surveys to determine, after the fact, where the outer end of the pilot hole may be.
An object of the present invention is to provide new and improved methods and apparatus for drilling and lining a borehole that extends underneath an obstruction and between surface locations on opposite sides thereof.
Another object of the present invention is to provide new and improved methods and apparatus of the type described where a lined borehole is formed with a single pass of equipment and instrumentation.
Another object of the present invention is to provide new and improved methods and apparatus of the type described where individual short sections of casing are joined end-to-end during a single pass operation, which gives the present invention great advantage in urban operations under crowded conditions.
Still another object of the present invention is to provide new and improved methods and apparatus of the type described where directional guidance is imparted to the drill bit throughout the hole forming operation so that the path of the borehole is very precise.